Formation of a semiconductor transistor typically comprises a doped substrate featuring a source and a drain bounded by dielectric regions, a gate dielectric layer, and a gate. One method of increasing the performance of that composition is enhancement of carrier mobility by introduction of a strained channel beneath the gate and between the source and drain.
The conventional approach to creating this strained channel is the replacement of a silicon substrate with bulk silicon-germanium, which is graded so as to produce relaxed silicon-germanium at the wafer surface. The silicon-germanium is then covered (capped) with epitaxial silicon. The difference in lattice constants between the silicon cap and the underlying silicon-germanium produces tensile stress in the silicon, and thus more carrier mobility in the silicon cap. A disadvantage with this approach is that the substitution of silicon substrate with relaxed silicon-germanium is both expensive and time-consuming. A further disadvantage is that dislocations in the silicon-germanium lattice are difficult to control and can spread into the strained silicon layer, degrading that layer and hampering performance. The higher the level of defect control, the more expensive the process becomes.
Another approach is capping a doped silicon substrate with an epitaxial layer of strained silicon-germanium. As in the conventional approach, the difference in lattice constants between the silicon-germanium and the silicon produces stress in the capped layer. Since the layers are reversed from the conventional approach, the silicon is relaxed while the silicon-germanium is compressively stressed. The strain produces the same benefit of enhanced carrier mobility. Unlike the conventional approach, this process is not expensive since the growth of a thin layer of stressed silicon-germanium is cheaper and less time-consuming than the growth of a thick layer of relaxed silicon-germanium. A disadvantage is that this method only improves p-channel metal oxide semiconductor (PMOS) performance, but degrades n-channel metal oxide semiconductor (NMOS) performance. An alternative is the use of a thin silicon-carbon layer instead of a silicon-germanium layer. The difference in lattice constants for the silicon-carbon on silicon configuration puts tensile, rather than compressive, stress on the silicon-carbon cap. However, this method only improves NMOS performance, but degrades PMOS performance.
It is desired to provide strained channels that improve different types of channels and devices, such as complementary metal oxide semiconductor (CMOS) devices.